Field of the Invention
The present invention relates to semiconductor devices such as diodes, transistors, thyristors etc. and more particularly to a semiconductor device having a high blocking voltage and a method for producing the same.
The semiconductor devices are classified into planar type, mesa type and bevel type, depending on the structure of PN junction formed between the P-type and N-type regions. In the planar type, the edges of all the PN junctions are exposed in one of the principal surfaces while in case of the mesa type the edges of the PN junctions are exposed in the etched-down surface in the periphery of one of the principal surfaces. In the bevel type, the edges of the PN junctions are exposed in the side surface connecting the two principal surfaces, the edges of the PN junctions intersecting the side surface perpendicularly or slantingly. The planar and the mesa type structures are suitable for the case where numerous pellets are produced from a large-area semiconductor wafer and these structures are preferred in small power semiconductor devices. On the other hand, the bevel structure is adapted for a power semiconductor device which must be provided with a substrate having a larger area.
The exposed edges of the PN junction are active and therefore easily affected by the atmosphere. For this reason, the exposed edges and their neighboring region are covered by insulating material for passivation. The insulator coating is referred to as passivation film. In the fabrication of planar and mesa type semiconductor devices, the passivation film can be formed before a large-area semiconductor wafer is scribed into numerous pellets, so that the formation of the passivation film is facilitated. In the production of a bevel type semiconductor device, the passivation film must be formed only after a semiconductor wafer is split into pellets. It requires highly specialized techniques and much labor to form a passivation film on the desired area of a small pellet with high accuracy. Thus, the formation of a passivation film in the fabrication of the bevel type device is much more intricate than in the fabrication of the planar and mesa type devices so that the efficiency of working the bevel type device is very low.
Now, the withstand voltages of the respective types will be considered.
In the planar type device, the PN junction has some portions bent almost at right angles, on which electric fields concentrate and since the regions on both the sides of the PN junction in the vicinity of the exposed edge of the PN junction are of high impurity concentration, the expansion of the depletion layer in the vicinity of the exposed edge is small. For these reasons, a semiconductor device of planar type, having a high blocking voltage is very difficult to fabricate. The blocking voltage of the planar type device is usually 300-400 V. In order to obtain higher blocking voltages with the planar structure, it is necessary to form a region called a guard ring, which encircles the exposed edge of the PN junction. The number of desired guard rings increases with the increase in the blocking voltage to be attained. Therefore, in the case where a semiconductor device having a high blocking voltage is fabricated in planar structure, there is incurred a drawback that the size of the device is too large.
The mesa type semiconductor device can blocking a voltage of about 600 V since it has no bent portion in the PN junction and since the impurity concentrations of the regions on both the sides of the PN junction near the exposed edges of the PN junction are lower than those in the planar type device. For the achievement of higher withstand voltage with the mesa structure, the area of the exposed surface of the intermediate high resistance layer must be increased or the depth of etching-down must be increased. These artifices, however, give rise respectively to a drawback that the resulting device has too large a size and a drawback that the current conduction area is decreased. Namely, the etched-down surface of the mesa type device is so slanted that the areas of the cross sections of the low impurity concentration region by planes parallel to the PN junction and having successively descending levels may gradually increase (negative bevel structure). If the PN junction has its edge exposed in such a slant surface, the smaller the slant angle becomes, the greater is the blocking voltage. To attain higher blocking voltages with the mesa structure, the slanted angle of the etched-down slant surface must be made smaller. If the angle is rendered smaller, the area of the slant surface becomes larger. This leads to the drawback that the size of the resulting device is too large, as in the case of planar structure. On the other hand, if the etching is too deep, a mass production method in which after a large-area semiconductor wafer has been provided with PN junctions, passivation films and electrodes, the wafer is scribed into numerous pellets, cannot be adopted. In the application of the mass production method, it is necessary that even after the required etching has been performed, the thinnest portions of the semiconductor wafer, i.e. parts connecting individual pellets with one another, should have a thickness of at least 50 .mu.. A semiconductor substrate with a smaller thickness cannot be used as a large-area wafer since it is easily warped and broken during various processes. It is necessary to increase the thickness of the wafer and especially of the high resistance layer so as to make the substrate rigid enough. The increase in the thickness of the high resistance layer is accompanied by the degradations of characteristics such as increase in internal power loss and decrease in response speed.
In the bevel structure, the side surface in which the edge of the PN junction appears is so beveled that the area of the cross section of the high impurity concentration region by a plane parallel to the PN junction may be large (positive bevel structure) and it is needless to reduce the slant (bevel) angle of the slanted (side) surface as in the mesa structure. Therefore, the blocking voltage can be increased without increasing the size of the device. However, in this case too, there is still left a drawback that the mass production method mentioned above cannot be employed.